The colors that are used to draw tee-style error bars for any
:high-value and :low-value
values in a chart-widget. The value should be a list of RGB objects
(see make-rgb) and/or symbols
that are globally bound to RGB objects. The default value is a list of
the symbol red.

If the value is nil, then all tees are drawn
in red. Otherwise, the first object in the chart-widget's sequence of
chart-objects is drawn with
the first color, the second object with the second color, and so on.
If there are fewer colors in the list than there are chart-objects,
then the drawing code will cycle back through the list of colors as
needed.